A Daily Telegraph sting which embarrassed a string of Liberal Democrat MPs, including Vince Cable and Paul Burstow, breached rules against using "subterfuge", it has been ruled.

The paper sent undercover reporters to try to expose alleged discontent among Liberal Dems ministers, including Sutton and Cheam MP Mr Burstow and Twickenham MP Mr Cable.

Yet, despite deciding the tactic was a breach of the subterfuge rules, the Press Complaints Commission (PCC) accepted the Daily Telegraph's decision to secretly tape senior politicians had produced material that was "in the public interest".

Business Secretary Mr Cable was stripped of media regulation powers after boasting to journalists posing as constituents at a surgery that he had "declared war" on Rupert Murdoch.

But the PCC ruled that the newspaper launched the "disproportionately intrusive attention" without sufficient reasons and upheld a complaint made by Lib Dem President Tim Farron.

It added it would issue fresh guidance over the acceptable use of subterfuge in the wake of the successful complaint about the so-called "fishing expedition".

Mr Cable caused a political storm when he told undercover reporters from the newspaper being in coalition was "like fighting a war" and that he could use the "nuclear option" of resigning.

He was also barred from ruling on the proposed takeover of BSkyB by Mr Murdoch's News Corporation and lost all such media regulatory powers to Culture Secretary Jeremy Hunt.

After the sting Mr Cable, in an exclusive interview with the Richmond and Twickenham Times, said "great damage" had been done.

"All my colleagues, of all parties, feel very strongly that some great damage has been done by this," he said.

In December, Mr Burstow featured in another Telegraph story, in which it was claimed he had been recorded saying: "I don't want you to trust David Cameron ... in the sense that you believe he's suddenly become a cuddly liberal.

"Well, he hasn't. He's still a Conservative and he has values that I don't share."

Kingston and Surbiton MP Ed Davey was also a victim of the Telegraph stings.

Mr Davey, a business minister, was quoted as saying he was "gobsmacked" by a decision to cut child benefits to for higher-rate taxpayers.

Around 200 people contacted the PCC after the stories were published in December and Commons Leader Sir George Young led complaints that the undercover methods undermined democracy.

Defending its tactics, the newspaper told the PCC that the operation had proved that Lib Dem ministers - as it had been informed by unnamed senior Conservative sources - "were not consistent in their private and public statements" about the coalition.

But the watchdog said the initial evidence was insufficiently strong to warrant the level of intrusion and that the ministers concerned were asked "to comment on a series of policy issues with the evident intent of establishing on which subject they might say something newsworthy".

Mr Farron said: "I am pleased that the PCC has upheld our complaint and defended the vital principle that MPs of all parties should be able to talk freely to their constituents in their constituency surgeries."